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$\alpha$-helices

As the polypeptide chain coils in a right handed manner, the CO and NH groups of residues $i$ and $i+4$ respectively form hydrogen bonds which stabilise the helix. All or most of residues in a helix are bonded in this way, making it a relatively rigid unit of structure with very little free space in its core. The helix makes a whole turn per 3.6 residues, and can have between 4 and around 50 residues.